Method of making titanium-dioxide rectifiers



Nov. 5 1957 1-. s. SHILLIDAY ETAL 2,812,273

' METHOD OF MAKING TITANIUM-DIOXIDE RECTIFIERS Filed June 9, 1955 Current Density, cumperes/in 25 2o |5 1o 5 o 2 3 4 Pofential, vblts v INVENTOR.

Theodore S. Shilliday Charles 5. Feet BY rthur E. Middlet Zha4- M A TORNEYS.

United States Patent METHOD OF MAKING TITANIUM-DIOXIDE RECTIFIERS Theodore S. Shilliday and Charles S. Peat, Columbus,

Ohio, and Arthur E. Middleton, Indianapolis, Ind., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Air *orce Application June 9, 1955, Serial No. 514,232

2 Claims. (Cl. 148-63) This invention relates to asymmetrical electrical conductors containing titanium dioxide. In particular, this invention relates to the pretreatment of the surface of titanium metal in the production of titanium dioxide rectifiers.

semiconducting titanium dioxide has been produced by oxidizing titanium metal in either oxygen or steam to produce an oxide film on the surface of the titanium. Although the semiconducting oxide film is commonly referred to as a titanium dioxide film, this film is composed principally of oxygen-deficient titanium dioxide and probably small amounts of the lower oxides of titanium. The semiconducting properties of this film have been attributed to the imperfect nature of the oxide crystal lattice. Rectifying contact to the oxygen-deficient titanium dioxide film is obtained by contacting the film with any of several known counterelectrode metals. The base 1 electrical contact for the rectifiers so produced may be made directly to the titanium metal bearing the semiconducting oxide film.

It has now been found that pretreating the cleansed titanium metal surface prior to oxidation with hydrochloric acid and permitting the surface to be air dried significantly decreases the reverse current without deleterious change in the forward current. This improvement in electrical properties is believed to result from chlorine ions that have diffused into the oxide film from the surface of the titanium metal during oxidation. In the preparation of the titanium dioxide rectifiers in accordance with this invention, the surface of the titanium metal sheet is cleaned to remove all oxide and foreign matter from the surface of the titanium. As is conventional, the titanium surface is polished with an abrasive paper and, after polishing, the titanium surface is consecutively rinsed in benzene, acetone, and water, fol lowed by a detergent wash and then rinsed in hot and cold distilled Water. The titanium sheet is then pickled in a hydrofluoric acid-nitric acid pickling bath, waterrinsed, and then air-dried. The thoroughly cleansed titanium metal is then contacted with hydrochloric acid. The surface of the titanium metal wetted with the hydrochloric acid is then dried in air at room temperature.

After the surface of the titanium metal has dried, the titanium metal is oxidized in any of the known ways to obtain a thin, adherent layer of the oxygen-deficient titanium dioxide film on the titanium metal. Among the methods of oxidizing the titanium metal to obtain a semiconducting titanium dioxide film the preferred oxidation procedure is to oxidize the titanium metal at a temperature ranging from about 700 C. to about 900 C. in an oxygen atmosphere at atmospheric pressure for a period of from about one to 20 minutes. Optimum oxidation is obtained at about 825 C. for a period of about seven minutes. Adherence of the oxide film to the titanium metal decreases as the oxidation temperature increases above about 900 C. At temperatures below about 700 C. oxidation times become excessively long and a higher resistant oxide tends to form.

The curve of the figure is illustrative of the currentv'oltage characteristics of a typical hydrochloric acid eated titanium dioxide rectifier wherein the counter electrode is bismuth. In this instance, the titanium dioxide rectifier was made in accordance with the above-described procedure. Specifically, after thoroughly clean sing the surface of the titanium metal in the manner described, the titanium was immersed in a 1.2 normal aqueous hydrochloric acid solution for a period of about five minutes and then air dried immediately preceding oxida-' tion. The hydrochloric acid treated titanium was then oxidized in an oxygen atmosphere at a temperature of 900 C. for a period of 10 minutes. Rectifying contact was made to the oxide film with a bismuth counterelectrode which was applied by conventional vacuum evaporation techniques. Measurements of the current-voltage values of the rectifier, as plotted in the curve of the figure, shows that the incorporation of the hydrochloric acid treatment in the processing of titanium dioxide rectifiers results in a rectifying device with a significantly lower current in the reverse direction without deleterious effect on the forward current.

While the particular embodiments of this invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention in its broader aspects. Thus, the particular cleansing procedure disclosed is not critical to the invention. Other cleansing procedures may be used so long as the surface of the titanium is cleansed of all foreign matter and oxides prior to the pretreatment with the hydrochloric acid and subsequent oxidation. The concentration of the hydrochloric acid may be varied without departing from the scope of this invention. The cleansed surface of the titanium need be contacted with the hydrochloric acid only a length of time sufiicient to wet the titanium surface. Preferably the time of contact with the acid should be only a few minutes although longer periods of time will not deleteriously affect the titanium. Further, it is understood that the invention is not limited to the specific forms herein described. The rectifying junction may be either an area-type contact between the counterelectrode and the titanium dioxide semiconductor or may be a point contact.

What is claimed is:

1. In the process of producing semiconducting titanium dioxide by the oxidation of titanium metal, the improvement in combination with said oxidation of contacting the titanium metal with hydrochloric acid prior to oxidation and then airdrying the metal prior to oxidation.

2. In the manufacture of titanium dioxide rectifiers having a counterelectrode in contacting relationship with a titanium dioxide semiconductor wherein said semiconductor is produced by the oxidation of titanium metal to produce a semiconducting titanium dioxide film on the surface of the titanium, the improvement in combination with said oxidation of contacting the titanium metal with hydrochloric acid prior to oxidation and then air-drying the contacted titanium metal prior to oxidation.

References Cited in the file of this patent UNITED STATES PATENTS 1,769,852 Ogden July 1, 1930 2,060,905 Smith Nov. 17, 1936 2,699,522 Breckinridge Ian. 11, 1955 

1. IN THE PROCESS OF PRODUCING SEMICONDUCTING TITANIUM DIOXIDE BY THE OXIDATION OF TITANIUM METAL, THE IMPROVEMENT IN COMBINATION WITH SAID OXIDATION OF CONTACTING THE TITANIUM METAL WITH HYDROCHLORIC ACID PRIOR TO TO OXIDATION AND THEN AIR-DRYING THE METAL PRIOR TO OXIDATION. 